Since 1992 we have observed the central parsec of the galactic center in K-Band. We mainly use our Speckle-Camera SHARP I on the NTT located at La Silla, Chile, and NAOS/CONICA at the VLT on Paranal, which are operated by the European Southern Observatory.
Our primary goals are to find outSo far, we have shown that at least two periods of star formation have occured. There are three sorts of stars in the galactic center:
The supermassive black hole in the centre of the Milky Way was
discovered as a bright non-thermal radio source in the 1970s and
termed Sagittarius A* (Sgr A*). Potential X-ray radiation by Sgr A*
was detected with the X-ray observatory Rosat in the 1990s. A reliable
identification of X-rays from Sgr A* was finally possible with the new
X-ray satellites Chandra and XMM, with their high spatial resolution
and sensitivity. The radio emission of SgrA* only varies slowly on
time scales of several days to a few hundred days and generally with
an amplitude <10%. However, in the X-ray regime, SgrA* was found to
exhibit two different states. On the one hand, in the quiescent state,
weak X-ray emission appears to come from a slightly extended area
around the black hole that appears to be evidence of hot accreting gas
in the environment of SgrA*. On the other hand, SgrA* shows X-ray
flares with a period of about one per day. During these flares, the
emission rises by factors up to 100 during several tens of minutes and
a distinctive point source becomes visible at the location of SgrA*.
The short rise-and-decay times of the flares suggest that the
radiation must origin from a region within less than 10 Schwarzschild
radii of a 3.6 million solar mass black hole.
Near-infrared flares from the black hole
Near-infrared high-resolution observations of the galactic centre (GC)
became possible since the beginning of the 1990s. Since then, the GC
stellar was regularly monitored by high-resolution NIR
imaging. However, in spite of all efforts, no unambiguous NIR
counterpart of SgrA* could be detected up to 2003. On the 9th of May,
during routine observations of the GC star cluster at 1.7 microns with
the CONICA/NAOS adaptive optics imager/near infrared camera at the ESO
VLT, we witnessed a powerful flare at the location of the black hole.
Within a few minutes, the flux of a faint source increased by a factor
of 5-6 and fainted again after about 30 min. The flare was found to
have happened within a few milli-arcseconds of the position of Sgr
A*. The short rise-and-decay times told us that the source of the
flare was located within less than 10 Schwarzschild radii of the black
hole.
Fig 1: Detection of NIR emission from Sgr A*. The images show raw AO images (60 s total exposure time) of an area 1''x1'' around Sgr A*, observed on May 9, 2003 UT. The left image was taken at the beginning of the observations, the right image about 40 min later. The flaring source is easily detected in the right image. Its position is offset -1.4±3.0 mas in R.A. and -0.2±3.0 in Dec. from the dynamical position of Sgr A* as it was determined from the orbit of the star S2. The star S2 is marked by a cross, the position of Sgr A* is indicated by a white circle.
During subsequent observations in 2003, we could observe more flares
from Sgr A* and also quiescent emission from a source at this
location. With hindsight, we could also detect a flaring source in
older, longer wavelength data from 2002. At the moment, we have
observed a total of four flares in the H, K and L-bands (1.7, 2.2 and
3.8 microns). The flares were observed at four different epochs within
a few milli-arcseconds of the location of SgrA*, which makes it highly
probable that they are indeed associated with matter in the immediate
environment of the black hole, which is also reflected in the very
short rise-and-fall time scales of the light curves. Independently,
flaring and variability of SgrA* in the L-band was also observed at
the Keck telescope by researchers from the University of California,
LA, in June 2003.
Fig 2: Light curves of the Sgr A* NIR flares in 2002 and 2003, observed with NACO/VLT. The L'-band flare on August 30, 2002, was only partially covered by observations. Gaps in the time series of the H-band flare on May 10, 2003, and of the KS-band flare on June 15, 2003, are due to sky observations and instrument failure, respectively. For comparison, the emission of the steady emission of the star S1 near Sgr A* is shown in all the plots (light grey data points). Arrows in the plots of the two KS-band flares indicate substructure peaks of the flares. Both KS-band flares show very similar quasi-periodicity, although the second flare was observed more than 24 h after the first one and must thus have been an unrelated event. The upper right panel shows the normalised power spectrum of the two KS-band flares. Both of them show a significant peak at a frequency corresponding to time scales of 16.8±2.0 min. In both cases, the power spectrum of S1 does not show such a frequency.
The quiescent and flaring NIR emission from SgrA* fills an important
gap in our knowledge of the spectrum of this source and will allow to
constrain the existing models on how the radiation is produced. While
the quiescent emission appears to be largely consistent with an origin
in the high-energy tail of a synchrotron spectrum, the mechanism of
the NIR flares is uncertain. Although the NIR flares were observed at
different epochs, they might hint at a blue colour of the flares which
would be a challenge to current theories. Simultaneous,
Multi-wavelength NIR and X-ray observations of the GC are planned for
the next year. The chances are high that these observations will
provide the required data to constrain the models and to establish (or
exclude) a relation between the X-ray and NIR variability.
Fig 3: Spectral energy distribution of the
emission from Sgr A*. This plot shows the extinction and absorption
corrected luminosities.
All error bars are ±1 sigma and include statistical and
systematic errors. Black triangles denote the radio spectrum of
Sgr A*. Open grey circles mark various
infrared upper limits from the literature. The three
X-ray data ranges are (from bottom to top) the quiescent state as
determined with Chandra (black; Baganoff et al., 2003), the autumn
2000 Chandra flare (red; Baganoff et al., 2000), and the autumn 2002
flare observed by XMM (light blue; Porquet et al., 2003). Open red
squares with crosses mark the de-reddened peak emission (minus
quiescent emission) of the four NIR flares. Open blue circles mark the de-reddened H,
KS, and L' luminosities of the quiescent state,
derived from the local background subtracted flux density of the
point source at the position at Sgr A*, thus eliminating the
contribution from extended, diffuse light due to the stellar cusp
around Sgr A*.
A spin measurement of the black hole?
The two K-band flares observed on the 15th and 16th of June 2003 are the flares that were completely covered by observations. Although they happened more than 24 hours apart and thus appear to be unrelated events, they both show a striking quasi-periodicity of the flare with a period of about 17 min. Of all possible periodic processes near a black hole (acoustic modes of a thin disk, Lense-Thirring precession, precession of orbital nodes, orbital motion), the period of matter circling the black hole near the last stable orbit is the shortest one. The observed period of 17 min is so short, however, that the only reasonable explanation is that the oscillations are produced by Doppler boosting of hot gas near the last stable orbit of a spinning (Kerr) black hole. The spin of the black hole will allow for a last stable orbit closer to the event horizon and thus with a shorter orbital frequency. From the observed 17 min period we estimate that the supermassive black hole Sgr A* has a spin that is half as big as the maximum possible spin of such an object.
Additional observations of flares and their quasi-periodicity will be needed in order to confirm this result. Should the quasi-periodicity indeed be an intrinsic feature of the flares then this will mean that the era of black hole physics has begun with the properties of black holes accessible to direct measurements!
